A highly conventional and widely used method of packaging utilizes a synthetic resinous film, which is disposed about the product and heat sealed to enclose it therewithin. In many instances, it is desirable to open the package by peeling apart the sealed films, and this must generally be done without destroying the integrity of the film itself. Accordingly, to provide a satisfactory package of this type, a balance of properties is necessary; i.e., the seal strength must be adequate to maintain the package in a closed and protective condition, while also exhibiting such release propeties as will permit opening by peeling when desired.
A variety of resins are commercially available, which provide excellent heat seals over practical ranges of operating conditions, typical of which are the copolymers of ethylene with vinyl acetate (EVA), methyl acrylate (EMA), and similar ethylenically-unsaturated esters. However, the bonds produced with such copolymers are of such a nature such that separation usually cannot be achieved without damage to the film.
It has previously been proposed to adulterate a heat sealing layer of, for example, EVA, with SURLYN ionomer, to thereby lower the strength of the seal so as to permit peeling at the interface. While somewhat successful, the approach has suffered from the serious disadvantage of causing stringiness, or "angel hair", to be produced upon separation of the films.
High molecular weight, crystalline, isotactic polybutylene resins (synthesized from butene-1 monomer), such as those which are commercially available from the Shell Chemical Corporation under the trademark WITRON, are known to have applications in combination with other polyolefins, such as polyethylene and polypropylene. Also, it is known from U.S. Pat. No. 4,075,290 that heat sealable polymer blends can be produced utilizing at least about 80 weight percent of polybutene-1 (described therein as a butene-1 polymer) with up to about 20 percent of ethylene/propylene copolymer, ethylene/butene-1 copolymer, or polypropylene. However, so far as is known, blends of heat sealable copolymers of the sort and with the properties hereinbefore described have not previously been proposed, in combination with a polybutylene resin or with any other polymer.
Accordingly, it is an object of the present invention to provide a novel synthetic resinous blend, which is capable of providing a peelable heat seal, and from which strings, or "angel hair", is not produced upon separation.
It is also an object of the invention to provide a novel film, and a heat sealed structure utilizing the same, which incorporate such a blend.